Thermographic transfer sheet

ABSTRACT

The heat-imaging of translucent plastic films for projection purposes using an imaged original sheet and infrared radiation characterized by the use of a transfer sheet having a heat-stable film foundation carrying a translucent heat transferable layer which is based mainly on a crystalline material carried by a resinous binder which softens and becomes adhesive during the heat-imaging process, the crystalline material being incompatible with the resin and providing the layer with a chalky appearance due to the refraction of light caused by its crystalline structure.

United States Patent Newman et al.

[451 Jan. 25, 1972 [54] THERMOGRAPHIC TRANSFER SHEET [72] Inventors: Douglas A. Newman, Glen Cove; Herbert Knabe, Centereach, both of NY.

Columbia Ribbon and Carbon Manufacturing Co., Inc., Glen Cove, NY.

[22] Filed: Mar. 16, 1970 [21] Appl.No.: 20,070

[73] Assignee:

Related US. Application Data [63] Continuation-in-part of Ser. No. 692,164, Dec. 20,

1967, abandoned.

[52] US. Cl ..1l7/36.2, l17/36.l, l17/l38.8 F [51] Int. Cl. ..B4lm 5/22, B41m 5/10 [58] FieldofSearch ..1l7/36.1,36.2

[56] References Cited UNITED STATES PATENTS 3,260,612 7/1966 Dulmage etal ..117/36.1

3,177,086 4/1965 Newman et al. ..1l7/36.l 1,781,902 11/1930 Gill ..117/36.2 3,436,293 4/1969 Newman. ..1 17/362 3,241,996 3/1966 Haas....'...... ....1 17/362 3,389,011 6/1968 Svensson ..l17/36.l

Primary Examiner-Murray Katz At!0rney.lohnson and Kline [57] ABSTRACT 6 Claims, 2 Drawing Figures 52- RECEPTOR LAYER CRYSTAL LINE REACTION IMAEES DONER LAYER r\\\\u PATENTEUJANZSISYZ I 3,631,414

.90- PROJECTION FILM A\\\\\\\\ CRYSTALLINE 3% DUPLICATE IMAGES LAYER m m HEAT STABLE ?'-Z0 TRANSFER SHEET WW 5% e //l\ l\ CRYSTALLRNE 3 v 33 REACTION mAeEs INVENTORS ,DougZas A. Newman rbcri' Knabe A 7' TUBA/E Y5 THERMOGRAPHIC TRANSFER SHEET This application is a continuation-in-part of parent application Ser. No. 692,164, filed Dec. 20, 1967, now abandoned.

The present invention is concerned with a specific area of the thermographic transfer imaging process. The thermographic transfer imaging process is one in which an original sheet carrying infrared radiation-absorbing images is superposed with a transfer sheet, carrying a heat transferable layer which does not absorb infrared radiation, and a receptor sheet, and infrared radiation is applied to cause selective heating of the original images and selective heating and transfer of corresponding portions of the heat transferable layer to the receptor sheet to produce a copy of the original sheet. In the reflex thermographic process the original sheet is positioned below the other sheets and the radiation is directed through the other sheets to the original, thus permitting the copying of opaque original sheets.

Most known thermographic transfer compositions are based upon relatively low melting point wax binder materials or combinations thereof with resinous materials to provide relatively low melting point compositions. Such compositions are generally acceptable for the imaging of single copies, hectograph masters and planographic plates since, although they tend to flow and spread slightly onto the receptor sheet, the loss in sharpness is not too noticeable to the naked eye.

However, the use of such compositions for the imaging of plastic films for projection purposes presents several difficulties. Since a plastic film has a smooth surface, the transfer composition tends to flow and spread more than when a porous paper receptor sheet is used. Also when the copy is projected onto a screen, it is usually greatly magnified so that the reduced sharpness of the copy images is greatly magnified and is clearly noticeable on the screen to the naked eye. A further difficulty stems from the amount of heat generated by the light source used to project the imaged film. Prolonged exposure during projection causes the buildup of ambient heat, which in turn causes the melting of the images being projected unless the images are so formulated as to resist melting at such temperatures.

Attempts to replace wax with resins in transfer compositions for projection purposes have been unsuccessful due to the fact that high-resin content layers are tough and resist transfer in image form to a plastic film. When the film is stripped from the transfer layer after exposure, the resin remains on the transfer sheet except for some spotty transfer to the film. The addition of conventional fillers such as clay reduces the toughness of the resinous layer but results in such an opaque layer, even when small amounts of clay are used, that the transfer sheet reflects infrared radiation to such an extent that it is useless in the reflex copying process.

The addition of plasticizers and/or waxes or waxy materials to resinous compositions results in a reduction of the toughness of the resinous composition but also causes a reduction of the melting temperature of the composition or ofa portion thereof so that the same problems are encountered as with wax transfer compositions.

The present invention is based upon the discovery of novel thermographic transfer sheets and compositions which are particularly well adapted for the reflex thermographic imaging of plastic films for projection purposes and which are free of all of the aforementioned disadvantages of prior known transfer compositions. The novel transfer sheets of this invention comprise a heat-stable plastic film foundation carrying a thin, semi-opaque, heat transferable layer of infrared radiation-transmissive resinous composition which contains a major amount by weight of a finely divided crystalline material and a minor amount by weight of a synthetic thermoplastic resinous binder material which softens and becomes tacky or adhesive at the temperature generated during the thermographic imaging process which generally ranges between about 180 F. and 220 F. The crystalline material and the binder material are both soluble in a common volatile solvent and the crystalline material is substantially incompatible with the binder material. The crystalline material does not melt or sublime to any significant degree at the softening temperature of the binder material and preferably has a melting temperature within the range of from 200 F. to 350 F.

The present invention is further illustrated by the drawings, in which:

FIG. 1 is a diagrammatic cross section, to an enlarged scale, of an imaged original sheet, a transfer sheet and a projection film superposed under the effects of infrared radiation but separated to illustrate the transfer of portions of the transfer layer to the surface of the projection film.

FIG. 2 is similar to FIG. 1 but illustrates another embodiment of the invention in which the projection film is provided with a thin translucent layer which is reactive with an ingredient of the transferred portions of the crystalline donor layer to form colored duplicate images.

The novel transfer layers of the present invention comprise a major amount by weight of a crystalline material, such as benzoic acid, salicylic acid, o-toluic acid or the like, and a minor amount by weight of a resinous binder material such as a butadiene-styrene copolymer or the like. The theory of operation of the present transfer layers is not completely clear. However, it is established that the crystalline material and the resin binder must be applied from a solution in which both materials are codissolved. When the solution is applied to an inert film foundation and the solvent is evaporated, the crystals form again within the resin binder, due to their incompatibility therewith, and appear to alter the toughness of the resin and render it more frangible. Possibly the crystals form between polymer chains of the resin and spread the chains to form a weaker, less cohesive, frangible resin structure.

Secondly, the present transfer layers are semi-opaque in appearance, due to the high content of light-refracting crystals, and yet permit sufficient infrared radiation to pass through to the underlying images to generate sufficient heat to soften the transfer layer in the areas overlying the images. lf sufficient heat is used that the crystals melt or sublime in these areas, on instantaneous exposure, an infrared radiation absorption differential is established and additional infrared radiation is able to pass through these clarified or transparent areas to heat the underlying images to an even higher temperature while the background areas of the transfer layer remain semi-opaque. This higher temperature insures the complete bonding and transfer of the heated areas of the transfer layer to the receptive film to form the projection transparency. Attempts to use high-melting crystalline materials such as silica or nonmelting pigments or fillers such as clay have failed and one of the reasons is the opaque nature of these materials. Thus little or no infrared radiation can penetrate through to the underlying original images.

Equally important is the fact that silica and clay are not soluble in common organic solvents and therefore cannot be codissolved with the resin binder during application of the transfer layer. Therefore the resin retains its original cohesive or tough nature to a large extent and does not transfer sharply or cleanly.

Whatever the cause, the present crystal-containing resinous compositions soften and transfer from an inert film foundation sharply and cleanly at the thermographic temperatures and deposit and resolidify on the projection film to form images which sharply correspond to the original images and which have the same semi-opaque, chalky color as the transfer layer.

The crystalline nature of the major ingredient of the present transfer layers appears to be important for at least two reasons. These crystals affect the structure of the resinous binder, as stated supra, to convert the resin to a form in which it is sharply and completely heat transferable and yet do not accomplish this by a plasticizing effect which would result in a clear plasticized resin and reduce the desired color contrast during projection. The present crystals are used in such amounts that they are incompatible with the resin binder, i.e., they are incapable of forming clear solid solutions therewith after being codissolved therewith in a common solvent and resolidified by evaporation of the solvent. In most cases the crystals appear to be completely insoluble in and complete nonsolvents for the resin. However, some degree of solvency is permissible provided that the crystals are present in amounts in excess of that amount which is soluble in or a solvent for the resin. The excess amount of crystalline material is incompatible with the resin binder and crystallizes on cooling to form the required semi-opaque light refractive layer. It is in this context that these materials are referred to as substantially incompatible.

Secondly, the preferred crystalline materials have a refractive index in the order of from 1.5 to about 1.6 and have a crystal structure which permits the passage of sufficient amounts of infrared radiation to an underlying image to cause softening of the transfer layer. Therefore thin coatings containing these materials appear semi-opaque or chalky and yet do not reflect excessive amounts of infrared radiation, whereby they are perfectly suited for use in the reflex imaging process to produce images which refract sufficient visible light to provide the desired contrast during projection.

The crystalline additives of the present invention which comprise from about 5 l to 90 percent by weight of the present transfer layers, may be any crystalline material which does not absorb substantial amounts of infrared radiation, which melts at a temperature within the range of from about 200 F. up to about 350 F. and recrystallizes on cooling, which is soluble in a volatile organic solvent which dissolves its resinous binder, which melts and recrystallizes at a temperature higher than the softening temperature of the resinous binder and preferably at least about 30 higher, and which is substantially incompatible with the resinous binder.

Preferred crystalline materials are the aromatic acids having melting points within the range of from about 200 F. up to about 350 F., such as salicylic acid, which melts at about 315 F. and sublimes at about 169 F., benzoic acid, which melts at about 252 F. and sublimes at 212 F. o-toluic acid, which melts at about 225 F., acetyl salicylic acid, which melts at about 270 F., and the like.

The resinous binder material, which comprises from about to about 49 percent by weight of the present transfer layers, may be any synthetic thermoplastic resin which has a softening point lower than the melting point of the crystalline additive and within the thermographic imaging process temperature range of from 180 F. up to 220 F., and which is tacky or adhesive when heat softened, and which is soluble in a volatile organic solvent which also dissolves the crystalline additive. Preferred materials are butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, polybutene resins, polyethylene, vinyl ether polymers such as poly (vinyl butyl ether), acrylic resins such as butyl acrylate and hexyl acrylate and copolymers of these with methyl acrylate. Other vinyl resins such as polyvinyl butyral and polyvinyl acetate, cellulose film formers such as ethyl cellulose and cellulose acetate and the like, are also suitable, particularly as blends with the preferred resins to produce binders having the required softening or melting temperature. The most preferred resins are butadiene-styrene copolymers available from Goodyear Tire and Rubber Company under the Trademarks Pliolite S730 and S5. The former is a 30 percent solution in toluene and contains about 70 percent styrene. The latter is a dry copolymer containing about 85 percent styrene.

The film foundation to which the transfer layer is applied should be one which is inert to the solvent used to apply the transfer layer and which does not distort or melt at the thermographic temperatures, i.e., up to about 450 F. Preferred films are polyethylene terephthalate polyester, polyvinyl fluoride, polytetrafluoroethylene, cellophane, polyvinyl chloride, and the like.

The present crystalline coatings are applied as solutions containing a major amount of volatile solvent, to which the film foundation is inert, so that after evaporation of the solvent the dry coating has a weight of from about one-half pound up to about 4 pounds per ream of 500 sheets which are inches by 38 inches in dimensions. The preferred dry weight is from about 1 to 2 pounds.

In use, the present transfer layers appear to mass transfer to the projection film substantially completely as illustrated by the drawings. Depending upon the thickness of the crystalline transfer layer 22 and the temperature and duration of exposure some amount of the layer 22 may remain behind on the transfer film 21 in the heated areas but this does not appear to have any material effect on the imaged transparency since the amount of crystalline material transferred to the projection film is sufficient to create an imagewise refractive barrier to the projection light. Thus the projected images appear black on the screen, and little or no variation is seen in the blackness of images projected from films imaged under different conditions of heat exposure and/or using different thermographic exposure machines of the belt or roller types.

However in all cases the thermographic imaging process must attain a temperature sufficient to tackify the resinous binder of the transfer layer 22 in contact with the projection film being imaged so that the transfer layer adheres to the projection film in the heated areas. After removal from the infrared radiation, the transfer sheet and the projection film are bonded together in the heated areas and must be stripped apart. During the stripping operation the heated portions of the transfer layer 22 remain bonded to the projection film in the form of crystalline lightrefractive images 31 corresponding to the original images 12 of the original sheet 10, as illustrated by FlG. l of the drawings.

The following example is given by way of illustration and should not be considered limitative.

EXAMPLE 1 The following solution is coated onto the surface of a 0.5

plied to evaporate the solvent to form a dry semi-opaque chalky layer having a weight of about 2 pounds per ream (25 inches X38 inches X500 sheets).

The novel transfer sheets of the present invention are used to produce projection transparencies of original subject matter which it is desired to enlarge by projection onto a screen for viewing. The original subject matter may be present on any original sheet in the form of infrared radiation-absorbing images, such as on the page of a book. As illustrated by FIG. 1 of the drawing, the imaged original sheet 10 is superposed with the transfer sheet 20 and the projection film 30 in the order shown and infrared radiation lamps 40 are illuminated against the projection film. A portion of the infrared radiation penetrates through to the underlying original images 12 where it is absorbed to generate an imagewise heat pattern which is conducted back to tackify corresponding areas of the transfer layer 22.

When the superposed sheets are removed from the influence of the radiation, the heated areas of the crystalline layer 22 resolidify against the surface of the projection film 30 and bond thereto. Next, the transfer sheet is stripped from the projection film and the resolidified portions of the transfer layer 22 remain bonded to the surface of the projection film and transfer thereto as duplicate images 31 which have the same semi-opaque appearance as the crystalline layer 22. In

the embodiment illustrated, images 31 are mirror-reverse duplicates of original images 12 so that the projection light is directed against the unimaged surface of the projection film in order to project images 31 as a correct-reading images.

it is preferred that the surface of the projection film softens in the heated areas to provide an exceptionally good bond with the softened areas of the crystalline layer. The preferred film is polystyrene but other films of materials which soften at the thermographic temperature, between 180 F. and 220 F., also provide good results. Films having higher softening temperatures may also be used since the present crystalline transfer layers also have good affinity therefor due to the adhesive nature of the binder material. However it is preferred to insure the formation of a strong bond between the film and images by providing such nonsoftening films with a translucent coating of a plastic composition which softens at the temperature attained during the thermographic imaging process, i.e., between about l80 and 220 F., as illustrated by FIG. 2 of the drawing.

in FIG. 2 the projection film 30 is a nonsoftening material such as polyethylene terephthalate polyester or other film listed hereinbefore in connection with heat-stable film 21 of the transfer sheet. The film is coated with a solution of a plastic material such as polystyrene, or other heat-softening resin, dissolved in a volatile solvent which is a nonsolvent for film 30, and the solvent is evaporated to leave a very thin continuous receptor layer 32 having a thickness preferably no greater than about 0.5 mil. Since the layer 32 is thin and translucent and preferably so clear that its presence is nearly undetectable, there is no interference with the projection operation and the film can be imaged and used in exactly the same manner as hereinbefore described in connection with the projection film of FIG. 1.

In still another embodiment, the receptor layer 32 on the projection film may contain a chemical which is reactive with the crystalline material of donor layer 23, or with a complementary chemical present in donor layer 23, to produce opaque, colored reaction images 33 on the projection film as shown by FIG. 2 of the drawing.

For instance, receptor layer 32 may contain a substantially colorless acid-sensitive dyestuff base such as any of those mentioned in U.S. Pat. No. 3,230,875, the acidic nature ofthe crystalline material transferred thereto, in the case of materials such as benzoic acid or the like, being sufficient to develop the dyestuff and cause color formation. This is a preferred embodiment since it does not require the presence of a separate color former in the donor layer. However, separate color formers may be added to the donor layer in an amount of up to about 10 percent based upon total weight of the crystalline layer provided that the color former is inert with respect to the crystalline material and the resin and does not absorb infrared radiation. Similarly the color-forming ingredient of the receptor layer must be one which does not absorb infrared radiation.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

We claim:

1. A thermographic transfer sheet comprising a heat-resistant synthetic thermoplastic film foundation having thereon a thin, solvent-applied, infrared radiation-transmissive, semiopaque layer of heat transferable composition consisting essentially of a minor amount by weight of a synthetic thermoplastic binder material having a softening point within the range offrom 180 F. to 220 F. and a major amount by weight of a crystalline aromatic acid which has a melting and recrystallizing point at least 30 higher than the softening point of said binder material and a refractive index between about 1.5 and 1.6 and which is substantially incompatible with said binder material and which is soluble in the application solvent which also dissolves the binder material.

2. A thermographic transfer sheet according to claim 1 in which the aromatic acid has a melting point between about 200 F. and 350 F.

3. A thermographic transfer sheet according to claim 2 in which the aromatic acid is salicylic acid.

4. A thermographic transfer sheet according to claim I in which the heat transferable composition comprises from about 10 percent to about 49 percent by weight of the binder material and from about 5l percent to about percent by weight of the aromatic acid.

5. A thermographic transfer sheet according to claim 1 in which the binder material is a butadiene-styrene copolymer.

6. A thermographic transfer sheet according to claim 1 in which the heat transferable composition comprises a substantially colorless color-forming material which is chemically reactive with an ingredient of a receptive plastic film to form a colored reaction product on transfer thereto, whereby the light-refracting duplicate images formed on the receptive plastic film are colored. 

2. A thermographic transfer sheet according to claim 1 in which the aromatic acid has a melting point between about 200* F. and 350* F.
 3. A thermographic transfer sheet according to claim 2 in which the aromatic acid is salicylic acid.
 4. A thermographic transfer sheet according to claim 1 in which the heat transferable composition comprises from about 10 percent to about 49 percent by weight of the binder material and from about 51 percent to about 90 percent by weight of the aromatic acid.
 5. A thermographic transfer sheet according to claim 1 in which the binder material is a butadiene-styrene copolymer.
 6. A thermographic transfer sheet according to claim 1 in which the heat transferable composition comprises a substantially colorless color-forming material which is chemically reactive with an ingredient of a receptive plastic film to form a colored reaction product on transfer thereto, whereby the light-refracting duplicate images formed on the receptive plastic film are colored. 